Download MEC Revision Electronics Photonics

MEC Physics review
Electronics & photonics
Themes
• Interpreting & decoding questions:
– Marks per question.
– Types of questions.
– Patterns in questions.
• Short answer questions:
– Using dot points.
– Idea per mark.
1 mark questions
‘06 Q6.
100
For 1 mark there are no calculations or interpretation to make.
Simply find the x axis value and read off the corresponding y
axis value.
Q9, ‘09
2 mark questions - Numeric
‘07 Q8.
750
1 mark for voltage divider equation and
correct substitution and 1 mark for final
solution of 750Ω. This question is a bit
mean since the finding of 1500Ω from 20
lux should be worth 1 mark as well.
Q1 ‘09
R2
VO = VI
R1 + R2
30
70
VO = 4.28V
VO = 10
2 mark questions - Graphical (draw)
‘07 Q1.
Voltage gain given as 80
1.6V
-1.6V
1 mark for inverting the waveform and 1 mark for multiplying
20 (and -20) by 80. Only 1 mark for the calculation as no
formula is involved.
2 mark questions - Short answer
‘07 Q9.
Decrease
- 1 mark
At increased lux RLDR will be lower. R must be lowered to
increase voltage drop across LDR to 4.0V.
- 1 mark
Make sure you answer the question explicitly. Make sure you
have 1 clear idea per mark after that.
3 mark questions - Graphical (read) + numeric
‘06 Q7.
1 mark for finding 400Ω from graph, 1
mark for voltage divider equation and
substitution, 1 mark for 3kΩ.
This is a fair allocation of marks.
3 mark questions - Numeric
Vout = Vin (
50Ω
Vout
20VRMS
150Ω
Vout
Vout
R2
)
R1 + R2
‘05 Q4.
1 mark for finding the equivalent
voltage divider circuit, 1 mark for
150
voltage divider equation and
= 20(
)
50 + 150 substitution, 1 mark for 15V.
Finding the equivalent circuit appears
150
= 20(
)
to be a lot of work for 1 mark but,
200
really, it’s maths you should be able
3
= ´ 20 = 15V to do in your head.
4
Q2 ‘09
V2
P=
R
but V = 10 - 4.28
V = 5.71
5.712
P=
40
P = 0.81W
Q3 ‘09
1
1
1
= +
Req R1 R2
1
1
1
=
+
Req 40 20
1
3
=
Req 40
Req =
40
= 13.33
3
voltage divider...
3 mark questions - Short answer
As light gets less bright resistance of LDR goes UP.
As resistance of LDR goes up, ratio RLDR:R goes UP.
As ratio goes up VOUT goes UP.
Make sure you have one clear idea per mark.
‘10 Q7.
‘09 Q11.
Increased
As T, R
Maintain Ratio
‘05 Q10.
4 mark questions - Graphical (draw)
I
I
V
V
1 mark for each graph and 1 mark for each set of correctly
labeled axes.
This is a reasonably simple question for 4 marks.
4 mark questions - Short answer
‘06 Q8.
Modulation occurs at P where the electrical signal from the
microphone is converted into a light signal of varying intensity
before being transmitted along the fibre.
Demodulation occurs at Q where the varying intensity light
signal is converted back into an electrical signal before being
passed to the speaker.
Modulation and demodulation involve the conversion of one
type of signal to another. Make sure you describe what form
the signal is being converted from and to.
Short answer
• Where most marks are lost (not gained).
• Often the difference between getting an
A and getting a B.
• Very easy to “muck up”.
Common errors
• Verbal diarrhea.
– Trying to say too much.
• You repeat yourself (waste of time and space)
• You contradict yourself (can lose marks)
• You write about irrelevant things
Common errors
• Copy from reference sheet.
– Relying on pre-prepared responses.
• Usually does not address the question
• Usually reads like a dictionary description
• Examiners know they are not your words.
Common errors
• Lack of detail in your response
– Too short
• Does not address criteria
• Does not show required depth of
understanding.
• Leaves Examiners asking “but why”.
Examiners reports
Numerical questions
• These have one correct answer.
• Usually have one best solution.
• Common errors:
– Not showing enough working.
– Consequential errors.
Strategy
• Find the appropriate formula and write
it out.
– This shows the examiner you know
which tool to use.
• Substitute immediately. Do not do
any algebra.
– This shows the examiner you know how
to use the tool
– Eliminates any chance you might stuff up
the algebra.
Final strategies
• 1. Pace yourself.
– How many marks in how many minutes?
– Plan for checking time at the end.
– Work on strategies to give you some “Oh
bugger!” time.
• 2. Easy marks first.
– Build confidence.
– Buy time.
Final strategies
• 3. Use data sheets wisely.
– Worked examples of hard questions.
– Reminders of your common mistakes.
– Not formulas – these are given to you.
• The best preparation is practice papers.
MEC Physics review
Electronics & photonics